Depending on the system setup, the TYPO3 CMS backend may be available
with an encrypted connection (SSL). If this is the case, you should
always use the secure, encrypted connection between your computer and the
TYPO3 CMS backend. This is done by using the prefix “https://” instead of
“http://” at the beginning of the website address (URL).

Under specific circumstances, a secure connection is technically
possible but an invalid SSL certificate causes a warning message. In
this case you may want to check the details of the certificate and let
the hosting provider confirm that this is correct. Please clarify with
the system administrator if an encrypted connection is available or
can be instituted.